Significant improvements in early post-transplant survival rates have been achieved with the introduction of newer pharmacologic immunosuppressive agents over the last decade. Nevertheless, the need for further refinement of available therapeutic protocols is emphasized by the morbidity and the annual attrition rate of 3-4% observed in currently treated allograft recipients primarily as a result of chronic allograft vasculopathy. The objective of this Program Project is to improve the outcome following heart transplantation by defining the essential conditions for and clarifying the mechanisms involved in donor-specific tolerance induction. The rationale linking the objectives of the 3 interrelated Projects and 2 Core Units is that since long-term donor-specific nonreactivity following withdrawal of immunosuppressive agents can be induced in many experimental models, including non-human primates, it should be possible to identify and reproducibly provide the immunologic perturbations which lead to this state in man. We have now established "proof of principle" of one approach, employing mixed chimerism, in five patients who received simultaneous kidney and bone marrow transplants from living donors. Our hypothesis is that definition of the mechanisms involved with such tolerance induction will lead to rational modifications of this initial conditioning regimen, making such approaches applicable to broader categories of patients, including recipients of cadaver donor organs. This would be of particular significance for heart allograft recipients in whom tolerance induction could prevent the accelerated atherosclerotic process that develops in at least half of otherwise successful transplants treated with conventional immunosuppression. The specific aims are to characterize in a continuum of models, including mice selected for specific incompatibilities or genetically engineered defects, partially inbred swine, and non-human primates, the mechanisms of action of conditioning regimens, with or without donor bone marrow transplantation, that induce tolerance. The methodology of the studies includes genetic manipulation, renal and cardiac allografts in murine and large animal models, flow cytometry, ELISPOT and functional analyses of direct and indirect pathways of reactivity. The relevance of such detailed studies to the treatment of allograft recipients has been demonstrated by previous successful translation of our observations to such clinical protocols, as monoclonal antibody treatment of rejection and an initial approach to tolerance induction. We anticipate ongoing progress will continue to contribute to a reduction in the morbidity and mortality associated with transplantation.